1. Field of the Invention
The present invention relates to an NOx detection apparatus connected to an NOx sensor which detects the concentration of NOx contained in a to-be-measured gas, and to an NOx sensor system.
2. Description of Related Art
A conventionally known NOx detection apparatus is connected to an NOx sensor configured by use of a solid electrolyte member, and is used to detect the concentration of NOx contained in a to-be-measured gas.
In such an NOx sensor, the to-be-measured gas is introduced into a first measurement chamber through a predetermined diffusion resistor, and the oxygen concentration of the to-be-measured gas is adjusted to a predetermined concentration by means of a first pumping cell which is composed of a solid electrolyte member and a pair of first electrodes. Next, the to-be-measured gas having the adjusted oxygen concentration flows from the first measurement chamber into an NOx measurement chamber, and NOx contained in the to-be-measured gas is decomposed by means of a second pumping cell composed of a solid electrolyte member and a pair of second electrodes, whereby a second pumping current corresponding to the NOx concentration flows between the pair of second electrodes.
The NOx detection apparatus detects the concentration of NOx contained in the to-be-measured gas on the basis of the second pumping current output from the NOx sensor.
Notably, in such an NOx sensor, which measures the gas concentration (NOx concentration) on the basis of an output (second pumping current) obtained via a pair of electrodes of the cell, the amount of the gas flowing into measurement chambers (the first measurement chamber and the NOx measurement chamber) is limited (controlled) by a predetermined diffusion resistor, whereby measurement is stabilized.
However, the conventional NOx detection apparatus has a problem in that the amount of the gas which flows into the measurement chamber changes in accordance with a change in the pressure of the to-be-measured gas in the vicinity of the attached NOx sensor, and the sensor output changes with the change in the pressure of the to-be-measured gas even when the gas concentration remains unchanged, and so the measured gas concentration involves an error.
Further, the NOx sensor has a problem in that the state of variation in the sensor output (output variation state) attributable to a change in the pressure of the to-be-measured gas differs among individual NOx sensors when the magnitude of the diffusion resistance for introducing the to-be-measured gas into the measurement chamber varies among the individual NOx sensors due to production variation, etc. of the individual NOx sensors.
To solve the above-described problem, there has been proposed a technique for suppressing a decrease in gas detection accuracy by means of setting pressure correction information in advance in accordance with the individual difference (individual characteristic) of each sensor and correcting the NOx concentration by use of the pressure correction information (see Patent Document 1). For example, the NOx concentration is corrected by use of, as the pressure correction information, Equation 2 which will be described later.